Sarcomere - Definition, Structure, Function and Quiz | biology dictionary (2023)

Sarcomere Definition

A sarcomere is the functional unit of striated muscle. This means that it is the most basic unit that makes up our skeletal muscles. Skeletal muscle is the type of muscle that initiates all of our voluntary movements. Herein lies the main purpose of the sarcomere. Sarcomeres are capable of initiating large sweeping movements by contracting together. Their unique structure allows these tiny units to coordinate the contractions of our muscles.

The image shows skeletal muscle fibers.

In fact, the contractile properties of muscles are a defining characteristic of animals. The movements of the animals are particularly smooth and complex. Skillful movement requires a change in the length of the muscle as the muscle flexes. This requires a molecular structure that can shorten along with the shortening of the muscle. These supports are found in the sarcomere.

Upon closer inspection, skeletal muscle tissue gives off a rough appearance called striae. These "streaks" are given off by a pattern of alternating light and dark bands that correspond to different protein strands. These striae are formed by the intertwined fibers that make up each sarcomere. Tubular fibers called myofibrils are the building blocks that make up muscle tissue. However, the myofibrils themselves are essentially polymers or repeating units of sarcomeres. Long, fibrous myofibrils are composed of two types of protein filaments stacked one on top of the other. Myosin is aPaufiber with a spherical head, and actin is afinestFilament that interacts with myosin when we bend.

A basic illustration of the underlying components of skeletal muscle up to the sarcomere is shown.

(Video) Structure & function of skeletal MUSCLES: Myofibrils, sarcomere, sliding filament theory.

sarcomere structure

Viewed under the microscope, muscle fibers of different lengths are arranged in a stacked pattern. Myofibril chains, ie, actin and myosin, form bundles of parallel filaments. When a muscle in our body contracts, it is understood that the way in which this occurs follows thatsliding thread theory. This theory predicts that a muscle contracts when the filaments are allowed to slide against each other. This interaction is then capable of resulting in a contraction force. However, the reason why the structure of the sarcomere is so crucial in this theory is because a muscle needs to physically shorten. Therefore, there is a need for a unit capable of compensating for the lengthening or shortening of a flexed muscle.

The sliding filament theory was first proposed by scientists who used high-resolution microscopy and filament staining to observe myosin and actin filaments in action at different stages of contraction. They were able to visualize the physical lengthening of the sarcomere in its relaxed state and shortening in its contracted state. His observations led to the discovery of sarcomere zones.

The figure shows the structure of a sarcomere. (Each zone is labeled).

They first observed that dynamic changes always occurred in the same places or areas. They found that a zone of repeating sarcomeres, later called the "A band," maintained a constant length during contraction. Band A has a higher content of thick myosin filaments, as expected from the stiffness of the band. The A band is the area in the middle of the sarcomere where the thick and thin filaments overlap. This gave the researchers an idea of ​​the central location of myosin. Within the A band is the H zone, which is the area composed solely of thick myosin. In essence, the A band can be assumed to encompass "all" myosin, including the myosin intertwined with actin in its bulbous head. At each end of the length of the sarcomere is the I band. The I bands are the two regions that contain only thin filaments. A quick way to remember this is that the I-Band has "thin active" filaments. The thick filaments are not far from the location of the I band; but on both sides their margins delimit where the thick filaments end. Similarly, the Z lines or discs that give sarcomeres a striated appearance under a light microscope actually delineate the regions between adjacent sarcomeres. The M line, or middle division, is right in the middle of the Z lines and contains a third, less important strand called myomesin.

Abbreviation for mental filament:

(Video) The Sarcomere- Structure and Components

  • I is a thin letter containing only thin wires.
  • H is a wider letter, it only contains thick filaments.

As mentioned above, contraction occurs when the thick filaments slide past the thin filaments in rapid succession to shorten the myofibrils. However, one key difference is that the myofilaments themselves do not contract. It is the sliding motion that gives them the power to shorten or lengthen.

sarcomere function

The sliding of the filaments creates muscle tension, which is possibly the main contributor to the sarcomere. This action gives muscles their physical strength. A brief analogy to this is the way a long ladder can be lengthened or bent as needed without physically shortening its metal parts.

Fortunately, recent research gives us a good idea of ​​how this slippage works. The sliding filament theory has been modified to include how myosin can pull on actin to shorten the length of the sarcomere. In this theory, the spherical head of myosin lies close to actin in an area called the S1 region. This region is rich in articular segments that can be flexed to facilitate contraction. The S1 curve could be the key to understanding how myosin is able to "walk" along actin filaments. This is done through the myosin-actin cycle. This is the binding of the myosin S1 fragment, its contraction, and eventual release.

When myosin and actin come together, they form stretches called "cross-bridges." These cross-bridges can be made and broken in the presence (or absence) of ATP. ATP enables the contraction of S1. When ATP binds to the actin filament, it moves it to a position that exposes its binding site to myosin. This allows the spherical head of myosin to attach to this site to form the cross-bridge. This binding causes the phosphate group of ATP to dissociate and thus is initiated by myosin.Power Center.Therefore, myosin enters a lower energy state in which the sarcomere can shorten. Also, ATP must bind to myosin to break the cross-bridge and allow myosin to bind to actin again and trigger the next spasm.

(Video) Structure of skeletal muscle and the Sarcomere


1. Which area of ​​the sarcomere maintains a constant length during contraction?
B.a band
D.Zone S

Answer to question #1

Bthis right. The A band is the region of the sarcomere composed mainly of myosin that maintains the same length during muscle contraction. However, it's important to remember that filaments never get shorter.

2. Which of the following contains only actin filaments?
A.a band
B.Banda H
D.z lines

(Video) The Sarcomere / Inter Biology / Sum Academy online Classes

Answer to question #2

Cthis right. As mentioned above, band I contains only "thin" filaments. Actin, in this case, is the designated thin filament within the sarcomeres/muscle tissue.

3. Which of the following contains only myosin filaments?
A.a band
B.Banda H
D.z lines

Answer to question #3

Bthis right. The H band contains only thick filaments. Myosin is the indicated thick filament and the filament that performs the attachment during sarcomere and thus muscle contraction.

(Video) Structure of Skeletal Muscle Explained in simple terms


  • Krans, Jacob et al. (2010). "The sliding filament theory of muscle contraction".pedagogy of nature 3. 3(9):66.
  • MH Education (2017). "Animation: Sarcomere Contraction".Human Anatomy: Mckinley O'Loughlin".Retrieved on 6/16/2017 from
  • Limitless (2017). "ATP and Muscle Contraction".Unlimited: the musculoskeletal system. Retrieved 2017-06-15 from -826-12069/


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